1. Field of the Invention
The present invention is directed to an electron emitter, and in particular to an electron emitter suitable for use as an electron source in an x-ray tube.
2. Description of the Prior Art
For generating x-radiation, conventional x-ray tubes have a cathode and an anode fused in a glass member opposite one another. The cathode has a helical or serpentine tungsten wire. This tungsten wire can be heated to emission temperature by applying a voltage, so that it is surrounded by an electron cloud. For generating x-radiation, a voltage can be applied to the cathode and to the anode, whereby the electrons are accelerated onto the anode where they convert their energy into heat and x-radiation. The surface of the anode onto which the electrons are incident is referred to below as the focal spot. Increasing the cathode-to-anode voltage results in all emitted electrons being ultimately accelerated onto the anode; the x-ray tube is thus driven into the saturation range. In this operating condition, the electrons are directly extracted from the tungsten wire that is thus projected onto the focal spot as the electron occupation. Since the tungsten wire is fashioned as a helix or in serpentine form, this leads to a varying electron occupation in the focal spot, as a result of which the x-radiation generated in the focal spot is likewise non-uniform. Upon transillumination of an exposure subject, this non-uniform distribution of the x-radiation is superimposed on the radiation shadow image of the exposure subject, which is undesirable.
German AS 11 49 115 discloses a cathode for an x-ray tube for generating a uniform electron occupation on the anode. To this end, this cathode is formed by an elongated, uncoiled glow wire that is convexly-arcuately shaped opposite the propagation direction of the electrons. A metallic shielding having a slot that accepts the glow wire is provided, with the front faces of the shielding which project beyond the glow wire in the direction of the anode being also convexly-arcuately shaped. Given this arrangement, high demands must be made on the electron-optical system in order to focus the emitted electrons onto the anode.
U.S. Pat. No. 3,833,494 discloses a cathode for an electrical discharge tube that has a high electron emission and a long useful life. This cathode is composed of a rhenium carrier on which a lanthanum hexaboride layer is cataphoretically applied and is sintered thereon. A large formation of rhenium boride during the operation of the cathode can lead to the rupture of the rhenium carrier, and as a result, the useful life of the cathode is reduced.
A glow cathode for an electron tube having high emissivity is disclosed by U.S. Pat. No. 4,752,713. This glow cathode is composed of a heat-resistant, metallic or ceramic member serving as a carrier and of a metallic activation substance that promotes the electron emission and that is composed of an alloy of a group VIII metal as well as of rhenium and of an element from the group of Ba, Ca, La, Y, Gd, Ce, Th, U, or is replaced by an intermetallic compound of the same elements. This activation substance covers the entire surface of the carrier and can be a lanthanum and platinum alloy.